/ Dictionary / Index R ricin: Meaning and Definition of Find definitions for: ri•cin Pronunciation: (rī'sin, ris'in), [key] — n. Chem. a white, poisonous, protein powder from the bean of the castor-oil plant. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: ricin (Thesaurus) richweed ricinoleic Related Content Daily Word Quiz: winsome Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language